This invention relates to switching power supplies or converters. Some embodiments use a switching power converter in a constant current or constant power mode.
Efficient and effective backlighting of displays is important for personal digital assistant computers (PDAs) and other portable computers, cell phones, cordless phones, handheld game devices, and the like. White light emitting diodes (LEDs) are popular for backlighting, but must be white as perceived by the human eye to enable color graphics, such as pictures, to be the correct color. LEDs of other colors may also require a certain power input for a specified color. In addition, many products require the ability to dim the display in response to ambient light, product mode (e.g. sleep, run, play a movie, etc.), user preference and the like. In some applications multiple LEDs are needed to uniformly illuminate a large display.
The color of some LEDs is affected by the current with which they are driven. For example, white LEDs actually emit a mix of blue and yellow wavelengths which human eyes perceive as white. The mix of the two wavelengths is affected by the current through the LED and the power delivered to the LED.
The current driver can be implemented as a switching power converter having an output terminal connected to the LEDs. The power converter keeps the voltage on the output terminal at a pre-calculated constant target value corresponding to the desired current. This voltage regulation to provide current control is not fully satisfactory however because the current through the LEDs can drift at a constant voltage due to heating of the LEDs, age, and possibly other conditions.
FIG. 1 shows a driver circuit that senses the LED current rather than output voltage. This is a buck converter circuit, described in “AN874 Buck Configuration High-Power LED Driver” (Microchip Technology Inc. 2003). Input voltage Vbatt is connected to one terminal of a PMOS switch 104 whose other terminal 110 is connected to induction coil L1. The other terminal of coil L1 is the driver circuit's output terminal 120, connected to the anode of LED 130. The LED's cathode is connected to resistor R8 whose other terminal is grounded. The LED's cathode is also connected to one terminal of feedback current sensing resistor R6. The other terminal 140 of resistor R6 is connected to the non-inverting input of operational amplifier 150. The inverting input of amplifier 150 receives a voltage Vdrive. The amplifier's output is connected to the gate of transistor 104. A smoothing capacitor C3 and a diode D are also part of the circuit. Amplifier 150 turns the switch 104 on or off depending on the current through LED 130.